1. Field of the Invention
This invention relates to solar cells in general and specifically to vertical multijunction solar cells.
2. Description of the Prior Art
In recent years the art of fabricating silicon solar cells has evolved from the first solar cells made by forming PN junctions in silicon by diffusing boron into N type silicon that had been previously doped with phosphorus, antimony, or arsenic. However, it was found that this type of device was radiation sensitive and degraded rapidly when used in space projects due primarily to the high energy electrons of the Van Allen Belts. A significant improvement over the first cells in radiation tolerance was achieved by switching to NP cells. These cells were generally fabricated by diffusing phosphorus into P type silicon that had been previously doped with boron or one of the other Group III acceptors.
Still further radiation tolerance might be achieved by the formation of a large number of parallel vertical PN junctions, as described in U.S. Pat. No. 3,617,825 issued to George E. Chilton, et al, and assigned to Computer Dial Corporation, Fair Lawn, New Jersey, entitled "Multijunction Photodiode Detector" although radiation tolerance was not mentioned by Chilton. It has recently been recognized by J. F. Wise of Wright Patterson Air Force Base (See Air Force Contract No. F33615-72-C-1310) that Chilton's structure, if made with very close junction spacing, would be more radiation resistant than a standard flat solar cell. Theoretical calculations by W. P. Rahilly, as well as by T. B. S. Chadda and M. Wolf, also performed under Air Force Contract No. F33615-72-c- 1310, suggest that the improvement in solar cell operating life in a typical space satellite mission will be 50% or even greater. The improved radiation tolerance of the vertical multijunction structure is due to the shorter average distance that a minority carrier must diffuse before being collected at the PN junction. This shorter distance that a minority carrier must travel also leads to a higher quantum efficiency, but this is a small effect compared to the enhanced radiation tolerance. The reduced carrier collection length means that for a given output power under a given level of solar illumination, the minority carrier diffusion length L can be considerably shorter than it would be in a standard solar cell. It is well known that high energy electrons reduce the minority carrier diffusion length. Thus, the vertical multijunction cell must be bombarded with significantly more damage producing high energy particles to reduce the cell efficiency by the same amount. However, the particular configuration of photodiode detector mentioned above suffers from severe degradation of minority carrier collection efficiency due to the very large number of surface intersections of the PN junctions. It is also quite difficult to fabricate in volume.
It is therefore an object of this invention to provide a method of fabricating a vertical multijunction solar cell having a minimum amount of exposed PN junction periphery.
Another object of this invention is to provide a method of fabricating a vertical multijunction solar cell having higher radiation tolerance than previously proposed vertical multijunction solar cells.
Another object of this invention is to provide a method of fabricating a vertical multijunction solar cell, which method lends itself to mass production techniques.